As an actual Southerner also, i cannot corroborate this. Tons of people i know freely use it online - facebook, twitter, reddit, etc. and in texts and groupchats. I'm not even in a very "country" region either, i'm in the middle of one of the biggest metro areas in the Southeast. Sure they aren't going to use it in a written report or a message to a boss, because you're correct that it isn't formal writing, but these things aren't formal writing. Nobody takes care to only write formally online. Why else do you think most people use text speak like "lol" or "hmu" as well as emojis?
And as an oldfag, i can confidently say that y'all has always been used occasionally on 4chan. I have never noticed a bump in its usage